This invention relates generally to measurement of capacitance, and in particular to measuring capacitance by delivering charge packets of known value to a capacitor of unknown value until a final voltage is determined, and then calculating capacitance based on the known total charge and measured voltage.
Capacitance measurement is an important feature of measuring instruments such as digital multimeters. U.S. Pat. Nos. 5,073,757 and 5,136,251, both of which are assigned to Fluke Corporation, disclose methods of measuring small and large capacitances in which an unknown capacitor was allowed to fully charge to a reference voltage at its RC rate, while at the same time a current proportional to the charging current was accumulated on the storage capacitor of a dual-slope integrating analog-to-digital converter (ADC). Small capacitances could fully charge in one integrate cycle of the ADC, while large capacitances required several integrate cycles to fully charge. In both cases, the proportional charge stored on the integrating ADC""s storage capacitor was removed during xe2x80x9cde-integratexe2x80x9d cycles over periods of time dictated by the amount of stored charge, and the time was measured to give an indication of capacitance value.
These prior art capacitance measurement techniques were unsatisfactory due to inordinately long measurement times because of the wait for the unknown capacitor to charge fully, leading to development of the capacitance measurement system disclosed in pending U.S. patent application Ser. No. 09/267,504, filed Mar. 12, 1999, wherein a constant current source was used to generate a linear ramp voltage across the capacitor being measured. This permitted measuring differential voltage (xcex94V) and differential time (xcex94T) and calculating capacitance from their ratio. While both measurement speed and accuracy were improved for a wide range of capacitors, it was a relatively slow process using a complex multiple-slope analog-to-digital converter to gather the needed parameters.
A problem with all these prior art methods is that since the value of capacitor is unknown, considerable effort is expended searching for the proper range for which measurements can be made. Also, particularly near the low end of a given range, capacitance values may be distorted because of resolution compression.
In accordance with the present invention, an apparatus and method of measuring capacitances are provided in which charge packets of known value are delivered to a capacitor of unknown value until a final voltage is determined, and the capacitance is calculated based on the known total charge and measured voltage.
Other objects, features, and advantages of the present invention will become obvious to those having ordinary skill in the art upon a reading of the following description when taken in conjunction with the accompanying drawings.